If an electronic circuit is supplied with power (i.e., if the power supply is switched on), some components may not attain predictable states. In order to prevent unpredictable states, it is established in the art to use power-on-reset (POR) circuits. These POR circuits are configured to provide and apply well defined reset signals to the electronic components, as for example to digital circuitry, until the power supply voltage has reached a safe operating value. However, although these stages have become more and more sophisticated and less power consuming, there is always a need for improvements.
Many conventional POR circuits consume power after the power supply has stabilized at an operating voltage. In battery powered systems, even a minor amount of DC power consumed by a POR circuit after system power-up can limit the battery operating time of the system. U.S. Pat. No. 5,323,067 discloses a POR circuit that is configured to switch the POR circuit off, after power-up. However, this known circuit requires many different stages which all have to be switched off separately. Furthermore, in order to achieve a reliable trip point of the POR circuit, dimensioning of the transistors used in the circuit is difficult.